Residential appliances such as heat pumps, air conditioners and refrigerators have traditionally used single-speed induction motors to drive compressors and fans. Traditional appliances regulate temperature by cycling the motor and associated compressor or fan between full capacity and zero capacity. This strategy meets only the average heating or cooling requirement, is inefficient, and regulates temperature comparatively poorly. Replacing the single-speed induction motor with an electronic adjustable speed drive (ASD) and a permanent magnet motor (PMM) greatly improves appliance performance. ASD-PMM based appliances operate continuously and regulate temperature by instantaneously adjusting the speed of the motor and the associated compressor or fan to match the current heating or cooling requirement. Both efficiency and temperature regulation are improved.
Traditional DC motors include stationary magnets, rotatable armature windings, and a mechanical commutator. While traditional DC motors with adjustable speed drive are known in the prior art, these motors are unsuited for many residential applications because the mechanical commutator is not able to operate in the refrigerant of the compressor of a heat pump, air conditioner, or refrigerator. This limitation may be overcome by using an electronically commutated electric motor which can be placed in the refrigerant eliminating the need for a rotating seal.
An example of an adjustable speed drive permanent magnet motor is disclosed in the Erdman U.S. Pat. No. 4,556,827. The Erdman '827 patent discloses an adjustable speed drive electronically commutated motor wherein the motor speed is regulated by chopping the rectified commutator input current at a frequency which is high with respect to the frequency of the input current. This method of motor speed regulation will be referred to as high frequency pulse width modulation (PWM). This high frequency chopping technique is required where rapid changes in motor speed are required, such as in a washing machine.
Liabilities of the high frequency pulse width modulation speed control system disclosed by the Erdman '827 patent include high cost, utility line distortion, and electromagnetic interference. The high frequency pulses generated by the commutator circuit generate electromagnetic interference (EMI) which is both conducted back to the utility line power source and radiated to the outside environment. The distorted line current drawn from the utility by prior art ASDs distorts the utility voltage causing a potential for interference with other nearby loads. The radiated EMI causes the potential for interference with electronic devices including communications devices. As a result, the high-frequency pulse width modulation technique used to regulate motor speed requires substantial filtering to reduce electromagnetic interference to acceptable levels.